1. Field of the Invention
The invention relates to an electric power steering system that assists a vehicle steering operation.
2. Description of the Related Art
Japanese Patent Application Publication No. 2011-46293 (JP 2011-46293 A) describes an electric power steering system that includes a steering mechanism that steers steered wheels of a vehicle in response to an operation of a steering wheel, and an assist mechanism that applies assist force to the steering mechanism. The assist mechanism is provided with various sensors that detect, for example, a steering torque applied to the steering wheel and a vehicle speed, and a controller that controls driving of a motor based on the values detected by the sensors. The controller sets an assist command value based on the steering torque and the vehicle speed, and controls driving of the motor such that a torque actually output from the motor follows the assist command value. Specifically, the controller computes a current command value corresponding to a target assist force, based on the steering torque and the vehicle speed. The current command value is a target value of a current to be supplied to the motor. The controller sets the absolute value of the current command value to a larger value as the absolute value of the steering torque is larger, or as the vehicle speed is lower. In a region in which the absolute value of the steering torque is smaller than or equal to a prescribed threshold, that is, in a so-called dead band region, the controller sets the current command value to zero irrespective of the magnitude of the steering torque. The controller controls driving of the motor by executing feedback control for causing an actual current value, which is a value of current actually supplied to the motor, to follow the current command value.
When the steering wheel is positioned near the neutral position, the steering torque applied to the steering wheel is considerably low. Thus, in the electric power steering system, the current command value is set to zero. That is, the motor is stopped, and thus assist force is no longer applied to the steering mechanism. In this situation, a driver's steering feel is strongly influenced by internal friction of the steering mechanism, for example, friction generated in a speed reducer that is disposed between the steering mechanism and the motor. Thus, the driver's steering feel is dominated by a sense of friction. This may be a factor of deterioration of the driver's steering feel.